I. Field of the Invention
The present invention relates generally to a protective assembly for capacitors and, more particularly, to a protective assembly for power capacitors.
II. Description of the Prior Art
In industrial applications, the industrial electrical load requires not only kilowatts (KW) or working current but also kilovars (KVAR) which are drawn by motors and other inductive electrical equipment which require magnetizing current. The kilovar requirement also increases proportionately with the inductive load.
In the well known fashion, the total apparent power requirement, measured in kilovolt-ampere (KVA) is the vector sum of the working and reactive power. Moreover, the power factor of the electrical apparatus represents the relationship between the working power and the reactive power. A power factor is equal to the working power divided by the reactive power and thus is equal to the cosine of the angle between the working power vector and the apparent power vector.
A low power factor caused by a high inductive electrical load is disadvantageous in that it results in poor electrical efficiency. Consequently, a low power factor results in power loss in the individual motor feeders and in step down transformers. A low power factor also increases the resistive heat losses in transformers and other electrical distribution equipment and also increases the difficulty in obtaining proper voltage stabilization. Perhaps more importantly, however, many electrical utility companies impose penalties on industrial users when which the power factor for the user falls below a prescribed amount. Consequently, low power factors can result in increased electrical utility bills.
In order to increase the power factor, it has been a previously known practice for industrial users to couple power capacitors in shunt with the inductive load since the capacitor reactance vector opposes the inductive reactance vector. Such power capacitors, furthermore, typically comprises an outer casing having three internal capacitors contained within it. Three capacitor terminals protrude upwardly from the top of the casing and one internal capacitor is electrically connected between each set of terminals. These terminals are then electrically connected with the user's inductive load to provide the necessary capacitive load to increase the power factor. These previously known power capacitors, however, have suffered from a number of disadvantages.
One disadvantage of the previously known power capacitors is that each internal capacitor within the power capacitor is thermally fused to its electrical terminal. Such thermal fuses, however, are prone to failure before the actual failure of the capacitor which results in expensive and unnecessary replacement of the entire power capacitor.
A still further disadvantage of these previously known power capacitors is that is is necessary to couple a bleed resistor across each pair of capacitor terminals to prevent the retention of excessive voltages within the capacitor once it is disconnected from the electrical apparatus. It has been the previous practice to directly connect a bleed resistor across the capacitor terminals for each of the capacitors. Consequently, the bleed resistors form a triangular or delta arrangement between the capacitor terminals on the top of the capacitor. In operation, however, such bleed resistors are known to break apart from the capacitor terminals or otherwise fail so that a portion of the failed bleed resistor physically contacts the power capacitor housing. When this occurs, the capacitor becomes shorted which destroys the power capacitor and also creates a safety hazard.
A still further disadvantage of the previously known power capacitors is that each capacitor terminal comprises a threaded shank onto which a nut is secured in order to form an electrical connection between both the bleed resistors and also the electrical leads from the power capacitor and to the inductive load. The nuts which are secured onto the capacitor terminals, however, must be accurately torqued within certain critical limits in order to ensure a good electrical connection to the capacitor terminals while simultaneously preventing damage and electrical shorts between the capacitor terminals. In practice, however, workman at the industrial plants where the power capacitors are used are both insufficiently skilled and do not have the proper tools to ensure the proper torque on the capacitor terminal nuts. As such, the improper installation of the previously known power capacitors has resulted both in premature failure of the power capacitors and also in improper operation of the power capacitors.
A still further disadvantage of the previously known power capacitors is that the electrical leads from the power capacitors, particularly when a plurality of power capacitors are utilized in a bank, must be wire wrapped together and fastened to the rack or housing in which the power capacitor are mounted. Such wire wrapping, however, is disadvantageous is that it is tedious and time consuming to perform.